A first generation of sound suppressor installations for jet engines, some of which having afterburners, were of the wet-cooling type. While their attenuation capabilities were somewhat acceptable, there were a number of disadvantages. With wet-cooling, the suppressor exhaust includes water vapor, raw fuel and free carbon when the after burner is fired. This was largely because the water spray quenched the flame. The unsightly vapor cloud created was not the biggest problem for it was the objectionable sooty vapor, or pollution, coating everything in the immediate vicinity. In addition, the sooty vapor had a deleterious effect on some types of acoustical materials.
Another suppressor design was sealed around the intake and exhaust of a particular aircraft type. The requirement for acoustic sealing created a requirement for accurate positioning of the aircraft relative to the suppressor. Even with careful positioning, some of the jet noise and inlet noise leaked around the seals between the aircraft and the sound suppressors. Because a large portion of the aircraft was not enclosed, objectionable noise levels were radiated from the airframe itself.
A recent European acoustical enclosure employs an acoustically treated augmentor tube which was sized so that the momentum flux of the aircraft's exhaust jet would pump in enough outside air through the mouth of the tube to cool the exhaust gases to thereby eliminate the need for water spray. Positioning of the aircraft is not difficult; both outside and inside sound levels are acceptable; the enclosures provided a lighted, all-weather, 24 hour-a-day place to work on the aircraft; the installations were designed so as to be adaptable to several aircraft types; and the problems attendant wet-cooling and sealing were avoided. However, inside the augmentor tube a limitation of this design quickly became apparent. The acoustic damping material in the walls of the tube, even though it was covered by a sieve-like plate, was battered apart by the roar of the jet exhaust and lost its noise absorption capability. This was largely due to the fact that the excessive vibration broke down the fibers and packed them together and, after a time, fused them in a mass. Additionally, the hot blast of the jet engines, particularly when on afterburner, melted the sieve-like retaining wall in the tube in an area downstream from the jet exhaust where the jet flow slowed. As a consequence, the tube had to be disassembled periodically and the fibrous sound absorptive material had to be replaced. Aside from the fact that this maintenance was expensive, there was a "down time" during which the enclosure could not be used.
Thus, there is a continuing need in the state-of-the-art for a jet exhaust noise suppressor that effectively attenuates noise and which is relatively maintenance free.